Due to the large number of output pins of semiconductor integrated circuits, it is desirable in some devices to allow an output pin to have multiple uses. That is, a pin may be used for multiple functions, the pin being multiplexed between the outputs of several circuits. Problems arise if two of the signals being driven do not have the same voltage amplitude. If an output signal is being driven from a power supply that has a larger voltage than power supplies for the other signals, the potential exists to force current into the lower voltage supplies. This will limit the voltage swing allowed on the output pin and have the potential to produce latchup in the integrated circuit.
The use of two power supples voltages of different voltage amplitudes is commonly used in integrated circuits, and the problem of isolating the circuitry which uses different voltages has been addressed in U.S. Pat. No. 5,408,147 in which two different power rails of different voltages is used. Disclosed is a first stage circuitry that is powered by a first voltage, and a second stage of PMOS transistors powered by a second higher voltage. Isolation is accomplished between the two differently powered stages by lining the PMOS transistors to the first stage through a series of controlling NMOS transistors. The PMOS transistors are completely turned off when necessary to avoid any undesirable conduction paths occurring due to the differences in the potentials of the two power rails.
Some circuits designed to overcome the problem of isolating the output pin from the power rail have utilized Schottky diodes to isolate the back gate of a PMOS output driver. Schottky diodes are not always present in every CMOS design. Also to prevent the PMOS device from conducting current in a MOS fashion, a second clamp PMOS device may be used. However, this device would have it's drain connected to the output pin with gate tied to the power supply and source connected to the gate of the output PMOS driver. The backgate of this clamp device would have to be isolated using Schottky diodes.